A Long-Term Enriched Environment Ameliorates the Accelerated Age-Related Memory Impairment Induced by Gestational Administration of Lipopolysaccharide: Role of Plastic Mitochondrial Quality Control

Abstract

Studies have shown that gestational inflammation accelerates age-related memory impairment in mother mice. An enriched environment (EE) can improve age-related memory impairment, whereas mitochondrial dysfunction has been implicated in the pathogenesis of brain aging. However, it is unclear whether an EE can counteract the accelerated age-related memory impairment induced by gestational inflammation and whether this process is associated with the disruption of mitochondrial quality control (MQC) processes. In this study, CD-1 mice received daily intraperitoneal injections of lipopolysaccharide (LPS, 50 μg/kg) or normal saline (CON group) during gestational days 15-17 and were separated from their offspring at the end of normal lactation. The mothers that received LPS were divided into LPS group and LPS plus EE (LPS-E) treatment groups based on whether the mice were exposed to an EE until the end of the experiment. At 6 and 18 months of age, the Morris water maze test was used to evaluate spatial learning and memory abilities. Quantitative reverse transcription polymerase chain reaction and Western blot were used to measure the messenber RNA (mRNA) and protein levels of MQC-related genes in the hippocampus, respectively. The results showed that all the aged (18 months old) mice underwent a striking decline in spatial learning and memory performances and decreased mRNA/protein levels related to mitochondrial dynamics (Mfn1/Mfn2, OPA1, and Drp1), biogenesis (PGC-1α), and mitophagy (PINK1/parkin) in the hippocampi compared with the young (6 months old) mice. LPS treatment exacerbated the decline in age-related spatial learning and memory and enhanced the reduction in the mRNA and protein levels of MQC-related genes but increased the levels of PGC-1α in young mice. Exposure to an EE could alleviate the accelerated decline in age-related spatial learning and memory abilities and the accelerated changes in MQC-related mRNA or protein levels resulting from LPS treatment, especially in aged mice. In conclusion, long-term exposure to an EE can counteract the accelerated age-related spatial cognition impairment modulated by MQC in CD-1 mother mice that experience inflammation during pregnancy.

Keywords: age-related memory impairment; enriched environment; hippocampus; lipopolysaccharide; mitochondrial quality control.

Figure 1

Timeline of experimental events. Pregnant mice were intraperitoneally injected with LPS or normal saline on days 15–17 of gestation (GD). Their offspring were weaned at post-natal day (PND) 21, and this mothers were divided into three groups based on whether the mice were exposed to an EE until the end of the experiment. MWM test was performed at 6 and 18 months of age. Fourteen days after the MWM test, mice were killed for subsequent biochemical experiments. CON, control group; LPS, lipopolysaccharide treatment group; LPS-E, Lipopolysaccharide plus enriched environment treatment group; MWM, Morris water maze.

Figure 2

Performances in the Morris water maze (MWM) test. Latency (A,D,G), average velocity (B,E,H), and distance (C,F,I) during the learning phase (trials 1–4); percent distance swam (J) and percent time spent swimming (K) during the memory phase (trial 5). Age and treatment had a significant effect on learning and memory performance in the MWM. Data are expressed as means ± SEM. *Significant differences compared with 6- or 18-month-old CON mice (*P < 0.05, **P < 0.01); ^#Significant differences compared with 6- or 18-month-old LPS-E mice (^#P < 0.05, ^##P < 0.01). 6: 6-month-old; 18: 18-month-old; CON, control group; LPS, lipopolysaccharide treatment group; LPS-E, LPS plus enriched environment treatment group.

Figure 3

Protein expression of Drp1, Mfn1, Mfn2, and OPA1 in the hippocampus. (A) Representative gel of Drp1, Mfn1, Mfn2, and OPA1 protein expression in the different groups. (B–E) Differences in Drp1, Mfn1, Mfn2, and OPA1 protein levels in the different groups. *Significant differences compared with 6- or 18-month-old CON mice (**P < 0.01); ^#Significant differences compared with 6- or 18-month-old LPS-E mice (^##P < 0.01). 6: 6-month-old; 18: 18-month-old; CON, control group; LPS, lipopolysaccharide treatment group; LPS-E, LPS plus enriched environment treatment group.

Figure 4

Relative mRNA levels of Drp1, Mfn1, Mfn2, and Opa1 in the hippocampus. (A–D) Differences in Drp1, Mfn1, Mfn2, and Opa1 in the different groups. *Significant differences compared with 6- or 18-month-old CON mice (**P < 0.01); ^#Significant differences compared with 6- or 18-month-old LPS-E mice (^#P < 0.05, ^##P < 0.01). CON, control group; LPS, lipopolysaccharide treatment group; LPS-E, LPS plus enriched environment treatment group.

Figure 5

Relative mRNA levels of Pgc-1α, Pink1, and parkin in the hippocampus. (A–C) Differences in Pgc-1α, Pink1, and parkin mRNA levels in the different groups. *Denotes significant differences compared with 6- or 18-month-old CON mice (**P < 0.01); ^#Denotes significant differences compared with 6- or 18-month-old LPS-E mice (^##P < 0.01). CON, control group; LPS, lipopolysaccharide treatment group; LPS-E, LPS plus enriched environment treatment group.

Figure 6

Protein expression of PGC-1α, PINK1, and parkin in the hippocampus. (A) Representative gel pattern of PGC-1α, PINK1, and parkin protein expression in the different groups. (B–D) Differences in Drp1, Mfn1, Mfn2, and OPA1 protein levels in the different groups. *Significant differences compared with 6- or 18-month-old CON mice (**P < 0.01); ^#Significant differences compared with 6- or 18-month-old LPS-E mice (^##P < 0.01). 6: 6-month-old; 18: 18-month-old; CON, control group; LPS, lipopolysaccharide treatment group; LPS-E, LPS plus enriched environment treatment group.